1. Field of Invention
The present invention relates generally to electrical power systems and more particularly, to a dynamic switch power conversion circuit for capturing the maximum power generated by a solar panel under varying sunlight conditions.
2. Description of Related Art
Solar power is a clean and renewable source of energy that has mass market appeal. Among its many uses, solar power can be used to convert the energy from the sun either directly or indirectly into electricity. The photovoltaic cell is a device for converting sunlight energy directly into electricity. When photovoltaic cells are used in this manner they are typically referred to as solar cells. A solar cell array or module is simply a group of solar cells electrically connected and packaged together. One of the drawbacks of the utilization of solar cells are their relatively expensiveness due to the high cost of production and low energy efficiency, e.g., 3 to 28 percent.
Prior techniques have been employed to improve the efficiency of solar cells. One of the earliest improvements was the addition of a battery to a solar cell circuit to load level the electrical output from the circuit during times of increased or decreased solar intensity. In itself, a photovoltaic or solar array can supply electrical power directly to an electrical load. However, the major drawback of such a configuration is the diurnal variance of the solar intensity. For instance, during daylight operation, a solar cell produces excess power while during nighttime or periods of reduced sunlight there is little or no power supplied from the solar cell. In the simplest electrical load leveling scenario, the battery is charged by the solar cell during periods of excessive solar radiation, e.g., daylight, and the energy stored in the battery is then used to supply electrical power during nighttime periods.
A single solar cell normally produces a voltage and current much less than the typical requirement of an electrical load. For instance, a typical conventional solar cell provides between 0.2 and 1.4 Volts of electrical potential and 0.1 to 5 Amperes of current, depending on the type of solar cell and the ambient conditions under which it is operating, e.g., direct sunlight, cloudy/rainy conditions, etc. An electrical load typically requires anywhere between 5-48 V and 0.1-20 A. To overcome this mismatch of electrical source to load, a number of solar cells are arranged in series to provide the needed voltage requirement, and arranged in parallel to provide the needed current requirement. These arrangements are susceptible since if there is a weak or damaged cell in the solar cell array, the voltage or current will drop and the array will not function to specification. For example, it is normal to configure a solar cell array for a higher voltage of 17 V to provide the necessary 12 V to a battery. The additional 5 V provides a safety margin for the variation in solar cell manufacturing and/or solar cell operation, e.g., reduced sun light conditions.
Since the current produced by solar cell arrays is constant, in the best of lighting conditions, the solar cell array loses efficiency due to the fixed voltage of the battery. For example, a solar cell array rated for 75 Watts at 17 Volts will have a maximum current of 75/17=4.41 Amperes. During direct sunlight, the solar cell array will in reality produce 17 V and 4.41 A, but since the battery is rated at 12V, the power transferred will only be 12*4.41=52.94 Watts, for a power loss of about 30%. This is a significant power loss; however, it is not desirable to reduce the maximum possible voltage provided by the solar cell array because under reduced sunlight conditions, the current and voltage produced by the solar cell array will drop due to low electron generation, and thus might not able to charge the battery.
FIGS. 1(a)-(d) illustrate Current-Voltage (I-V) and power electrical behavior outputs of a conventional solar cell module under different sunlight intensities and conditions. The current in milliamperes (mA) and the power in milliwatts (mW) are plotted on the vertical y axes. The voltage in volts (V) is plotted on the horizontal x axis. These figures show the shortcomings of the prior art in providing electrical load leveling for a typical 12 V battery connected to a solar cell array for energy storage during the daylight hours of sunlight whether full sun or not.
Six different I-V curves are shown in FIG. 1(a). Three of the curves are for a crystalline solar cell and another three of the curves are for an amorphous silicon module. The solar intensity falling on the arrays are labeled as 50, 75, 100, and 200 Watts (W) per square-meter (W/m2). The “Battery Charging Window” is illustrated by the two parallel slightly curved lines moving up from 11 and 14 volts on the x axis.
Also illustrated in this figure is the case where the lowest intensity I-V curves at 75 W/m2 enter slightly or not at all the “Battery Charging Window,” thereby resulting in little or no charging of the battery. This would be the case for heavily clouded or rainy days. Also shown is the result that some of the charging of the battery takes place to a lesser degree from the moderate intensity at 100 W/m2 depending on the type of solar cell array. This would be the case for semi-cloudy days. Finally, the condition for a high intensity flooding of the solar cell array at 200 W/m2 is shown. This would be the case for full sun days. In effect, FIG. 1(a) shows that the charging of a battery directly from solar cell arrays may not yield an optimum result depending on the type of solar cell array used and the conditions of the solar environment to which the solar cell array is exposed.
Industry standard crystalline solar cells are only effective at charging a 12 V battery at the highest intensity of 200 W/m2. Also, the amorphous silicon module, which is one of the most efficient present day solar cell arrays, although providing more charging power to the battery at all but the lowest of intensities, still indicates a significant fall off in power due to a decrease in current from the highest to the lowest solar intensity. So even for the most efficient solar cell modules available today, optimum power is still not being delivered to the battery.
A Maximum Power Point Tracker (MPPT or “power tracker”) is an electronic DC-to-DC converter that optimizes the match between the solar cell array and the battery. A MPPT can recover some of the power loss, provided that the power consumed by the MPPT circuitry is not excessive. In the example of the solar cell array outputting 75 W at 25 V (3 A maximum) described above, the addition of a MPPT circuit reduces the voltage output of the solar cell array to 13 V. Assuming the power consumed by the MPPT is minimal, the DC-to-DC converter conserves the 75 W of output power, and thus the output of the DC-to-DC converter is 13 V, 5.77 A (from conservation of power 25 V×3 A=13 V×5.77 A). Accordingly, the current produced is higher with the MPPT than the maximum current of the solar cell array without the MPPT. The reason for the use of 13 V is to provide a positive one Volt difference between the output of the MPPT circuit and the battery. However, an MPPT circuit requires a minimum voltage and power to operate. For instance, the minimum input requirements of a typical MPPT circuit available on the market is 19 volts at 50 watts of power. Other MPPT circuits require higher input voltages and powers. Thus if the voltage drops below 19 volts the MPPT circuit does not operate.
The challenge with using solar cell devices is that the power generated by these devices varies significantly based on both the exposure to sunlight and the electrical load applied to the device. A maximum current can be achieved with a short circuited load, but under this condition, the output power generated by the solar cell device is zero. On the other hand, if the load has a maximum voltage, the current derived from the solar cell device drops to zero, and then again no power is generated. Therefore, in order to yield maximum power the output load has to be adjusted based on the exposure level of the solar cell array to sunlight.
The sunlight conditions are often controlling on the performance of a solar cell array. A few notable conditions are illustrated in FIGS. 1(b)-(d).
FIG. 1(b) shows the electrical behavior of a 12 W flexible solar panel array under the conditions of low sunlight exposure levels due to an early morning indirect sun or an open sun at high angles of incidence to the array. Designated by the left vertical axis is the solar array output power in milliwatts and designated on the right vertical axis is the solar array output current in millamperes. The voltage output of the solar array is designated on the horizontal axis. As illustrated by the data plotted, the power and current outputs for this particular solar cell array cannot generate power to charge a 12V battery within the boundaries of the given lighting conditions. Power is available in excess of 10% of array capacity, but in order to make use of this power, a 12V battery cannot be used as in this example.
FIG. 1(c) shows the electrical behavior for the same 12 W flexible solar panel, but, in this case, under the conditions of increased sunlight illumination, but not full sunlight. It can be readily seen from this figure that the maximum power that may be obtained under these conditions is 8.65 W at 9.5 V, but it is commonly known that 13.5 V is necessary to charge a 12 V battery. At the required 12 V, the power available drops to 6 W, a reduction of 31% in the available power.
FIG. 1(d) shows the electrical behavior for the same flexible solar panel under exposure to full sun. In this case, the maximum output is 5.177 W at 16 V. However, the power available at 12 V is only 4.4 W. This is a reduction of 18% of the available power. The maximum voltage available is 16 V even though this flexible solar panel was originally designed for operation at 12 V.
With the exclusion of the highest sunlight intensities, the above examples show the deficiency of the prior art in matching the charging power requirements for a conventional 12 V battery. Accordingly, there is a need to efficiently capture the power of a solar cell during low power output due to, for example, reduced sunlight conditions.